1. Field Of The Invention
This invention relates to an electronic system for controlling the speed of a shaded-pole motor. The present invention is designed for use in acoustic devices producing pulsato and other musical effects.
2. Prior Art
In an acoustic pulsato system for producing the fast or full pulsato speed, the acoustic rotor is rotated at about five to eight revolutions per second and for slow operation the rotational speed of the rotor is about one-half to one revolution per second. One prior art system for producing pulsato is described in U.S. Pat. No. 3,245,284 which discloses a main motor for driving a rotor at full pulsato speed, and a secondary motor having an axially floating rotor that is spring-biased normally to be out of alignment with its stator, which when energized pulls the floating rotor into alignment with the stator by solenoid action. This movement is utilized to provide a releasable frictional coupling between the shaft of the secondary motor and a friction wheel carried on the main motor shaft. When the secondary motor is energized and the main motor deenergized, the acoustic rotor is driven at the slower speed of the secondary motor. A disadvantage of this system is that when the main drive motor is turned off following operation at full pulsato speed, the rotor slowly coasts to a stop without any braking effect, and produces undesirable droning sounds. This is overcome by the system described in U.S. Pat. No. 4,198,880, which also uses two motors but which provides automatic braking from full pulsato speed to either "slow" speed or to a completely stopped condition. The motors employed in both systems are preferably of the shaded-pole type, but the fact remains that two motors are required to provide "slow" and "fast" operation.
Other pulsato producing systems are also known in which an acoustic rotor is belt-driven by a single drive motor and adjustment of pulsato rate between "fast" and "slow" is accomplished with multi-step pulleys. Such systems suffer the disadvantage that different sets of pulleys are required for different line current frequencies.
In addition to the above, shaded-pole motors in general have heretofore been controlled by changing the impressed voltage in response to shaft speed information derived by means of a tachometer. This prior art system suffers the serious defect of taking a long time to slow down; thus, if used in an acoustic pulsato system, the earlier-mentioned undesirable droning sounds would be produced when going from "fast" to "slow" operation, or from "slow" to stop.
Another prior art system is disclosed in U.S. Pat. No 4,348,625 which teaches dual feedback means interactively connected to the gate electrode of a triac in the AC current line of a shaded-pole motor. One feedback loop controls speed through a frequency/phase detector to adjust the time delay of triac gating relative to the zero crossing points of each half-cycle of AC voltage until the tachometer sensed speed of the motor corresponds to a desired speed set by a voltage controlled oscillator. The feedback loop adjusts the time delay of the triac gating relative to the zero crossing points of every alternate half-cycle of AC power and decelerates the motor by an impressed half-wave pulsed DC current. One significant drawback of this system is that it does not provide error signals frequently enough to integrate over the time period and generate a control signal for the motor. Therefore a large time constant in the integration portion of the system is required which results in a tendency of the system to "hunt". This effect is commonly described in a feedback type of circuit as having the system overcompensate in one direction and then correct by under compensating in the other direction and accordingly oscillate about a particular value or point.
It is a general object of the present invention to provide a feedback loop for controlling the speed of a horn and a separate feedback loop to control the speed of a drum to produce pulsato or other musical effect.
It is another object of the present invention to provide a braking circuit for the drum to prevent undesirable distortion in the audio.
It is a specific object of the present invention to provide a feedback loop for a horn including a first pulse width modulator circuit which modulates a tachometer signal from the motor with a DC reference signal to provide a pulse having a width proportional to the motor speed and a second pulse width modulator circuit which modulates the integrated signal from the first pulse width modulated signal with a full wave rectified signal from the AC power line to provide a control pulse to adjust the time delay of a triac gating to adjust the motor speed.
It is a specific object of the present invention to provide a feedback loop for a drum including a first pulse width modulator circuit which modulates a tachometer signal from the motor with a DC reference signal to provide a pulse having a width proportional to the motor speed and a second pulse width modulator circuit which modulates the integrated signal from the first pulse width modulated signal with a full wave rectified signal from the AC power line to provide a control pulse to adjust the time delay of a triac gating to adjust the motor speed.
It is another specific embodiment of the present invention to provide a brake circuit for use with the drum to decelerate the drum when slowing or stopping.